This invention relates to a moisture detector and, more particularly, to the detection of a low percentage of water in the oil housing or stator housing of an electrical submersible pump wherein due to the motion imparted to the oil by the rotating shaft of the pump, the oil/water mixture is in the form of an emulsion.
Electrical submersible pumps have applications in many areas. For example, construction sites, mines, trenches, marine salvage, public works and sewerage treatment. Obviously, water must be prevented from reaching the motor housing of the submersible pumps. To this end, the pumps are generally provided with a plurality of seals which segregates oil in the oil housing from the motor housing and the fluid being pumped. The central rotating shaft of the pump maintains the oil in the oil housing in constant motion. If water were to somehow penetrate the lower seal arrangement and enter the oil housing, the oil/water mixture would become an emulsion apparent by its milky-white color.
In order to determine if any water has penetrated the oil housing, it is common practice to provide an inspection screw which, when removed, allows visual inspection of the oil in the oil housing. This requires removal of the pump from its operating environment and removal of the inspection screw. If a milky-white mixture is observed, it would be apparent to the observer that water has entered the oil housing. It is to be noted, however, that the amount of water in the oil housing cannot be determined from a visual inspection alone.
It has been deemed desirable to provide an automatic device for detecting the presence of water within the oil or stator housing of the pump; however, solutions to this problem have been restricted by certain agencies, such as the Underwriters Laboratories, which have limited the amount of voltage that can be applied to certain devices such as electric submersible pumps. The situation is further complicated by the fact that due to the motion of the oil, water in the oil forms discrete bubbles, each bubble electrically insulated from other bubbles by the oil itself which exhibits dielectric characteristics.
Due to the low voltage requirement, various low-voltage arrangements have been employed, one of which is the provision of a thermistor in the stator housing of the pump. Thermistors are devices which exhibit a change in resistance proportional to a corresponding change in temperature. Therefore, it can be seen that even if oil alone were to enter the stator housiing and make contact with the thermistor, the temperature of the thermistor would decrease and an alarm generated. However, it is well known that a certain amount of oil alone in the stator housing will not damage the motor, but will be burned away. Further, the presence of oil alone in the stator chamber is not uncommon since each time the pump motor is started, the mechanical elements may jump and the seals lifted, thereby allowing a small amount of oil to enter the stator chamber. This results in the generation of an alarm due to the cooling of the thermistor by the oil alone. Since, as stated above, a small amount of oil in the stator chamber represents no great danger, an alarm generated, due to the presence of oil alone, constitutes not only a nuisance, but also a severe reduction of the integrity of the detection system itself. Further, due to the low voltage requirement, it is necessary to provide complex and expensive electronic amplifying components. By adding additional components, the mean time between failure of the detection system is substantially decreased. A further disadvantage of the thermistor approach resides in the sensitivity of the thermistor to thermal ambients in the operational environment of the electrical submersible pump.
A second approach of detecting water in the stator housing of the pump is the basic conductivity approach wherein two steel electrodes are permanently provided in the stator housing. A voltage, typically 12 volts to 24 volts, is provided across the electrodes. When the water content becomes high enough, a conductive path will be formed between the two electrodes, and an alarm signal generated. This arrangement suffers from the same disadvantages as the thermistor approach due to the low voltage requirement, i.e., the requirement of additional complicated electronic amplifying apparatus. Further, before a signal will be generated (before a conductive water path between the electrodes is formed), it is necessary that a very large percentage of water be present, at which time, damage may have already been caused. In an attempt to reduce the percentage of water necessary to generate an alarm, the spacing between probes can be greatly reduced. However, this may lead to accidental conduction between the probes, thereby providing a nuisance effect and severe degradation of the reliability of the detection system.
As stated previously, the conductivity probe is build into the pump, and any malfunction would require disassembly of the pump to retrieve the probe. Further, a wire to each probe is necessary, and as the length of the wires increase, the amount of voltage drop due to the internal resistance of the wires is increased. This reduction in voltage across the probes necessitates the presence of a larger percentage of water in order to form a conductive path between the electrodes and thereby produce a warning signal.
There has been a great deal of discussion regarding the amount of water in the oil housing of the pump which can be tolerated before the situation is deemed critical. It has been stated by some that there must be 50 percent water in oil before the situation becomes critical, while others maintain that a percentage as low as 5 or 10 percent is critical. Irrespective of this, it would be greatly advantageous to be able to detect a very small percentage of water in oil as an indication that a leak does exist, thereby warranting closer surveillance or repair at a very early stage.
Ignoring the other disadvantages of the basic conductivity approach, it will, in all likelihood, provide an alarm or warning if the concentration of water in oil becomes very high. However, at this point, severe damage may have already occurred. Neither the basic conductivity probe approach nor the thermistor approach is suitable for detecting low percentages of water in the oil or stator housing of an electrical submersible pump.